media: dvb_demux.h: add an enum for DMX_STATE_* and document

kernel-doc allows documenting enums. Also, it makes clearer
about the meaning of each field on structures.

So, convert DMX_STATE_* to an enum.

While here, get rid of the unused DMX_STATE_SET.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>